Electric furnace



July 28 1942 "L. R. 'rrrcoMB 2,291,007

' ELECTRIC FURN-ACE Filed Feb, 7, 1941 2 sheets-sheet -1 July 28, 1942l.. R. TwcOMB 2,291,007

` v ELECTRIC FURNACE Fi 1ed Feb. '7, 1941 2 Sheets-Sheet 2 .ya y J4!mmm/.J7

Patented July 28, 1942 UNITED STATES PATENT OFFICE ELECTRIC FURNACE LecB. Titcomb, Chicago, lll. y Application February 7, 1941, Serial No.317,888

(Cl. 13Z3) g 2 Claims.

The present invention relates to electric furnaces adapted for heatingsalt baths used in the heat treatment of steels and other metallurgicalprocesses.

It is well known in the art that steel articles, such as tools and dies,may be tempered by heating the articles in a furnace and subsequentlydipping them in a `quenching bath. This method is not entirelysatisfactory since in many instances corners of articles, such as toolswill be overheated and the larger masses thereof underheated. As aresult of this objection, it has become customary in the art today toheat treat articles by immersing them in a hot salt bath, which ismaintained at a desired temperature, so that by leaving the articles inthe bath a suflicient lengthof time they will be brought to thetemperature of the bath and will be uniformly heated throughout theirentire masses.

These salt baths, 'of which sodium cyanide is an example, are generallymaintained at the desired temperature by disposing electrodes in thebath and passing an electric current between them and through the bathso that the bath itself constitutes the principal resistance element ofthe circuit for heating the bath. While this principle is not new, theelectrical resistances at room temperatures,'of nearly all salts whichare useful in metallurgical processes, are so high that for allpractical purposes the salts are substantially non-conductive. In orderto pass current through such asalt bath the salt must first be broughtto a fairly high temperature. Prior to my invention this has beenaccomplished by melting a 'portion of the salt'bath by some externalmeans,

such as a gas flame or carbon bridge disposed across the electrodes atthe surface of the bath. Whena sufiiclently large puddle has beenformed, a current capable of continuing and maintaining the meltingprocess will flow between the electrodes and after the expiration ofconsiderable time the bath will be brought to operating temperature.

This method of starting prior furnaces-is an inconvenient and timeconsuming process and therefore objectionable.

It is the primary object of my invention to provide an electric furnacehaving means for bringing the salt bath to proper operating temperaturein a relatively short time.

A further object is the provision of means for heating the salt bath toa temperature sufficient to permit the flow of normal load currenttherethrough after which the means referred to is automatically renderedinoperative.

Inorder to attain the above objects, I propose to incorporate anelectrical resistance element within one or more of the electrodesdisposed within the salt bath and cause an electric current to flowthrough the resistance element which in turn will heat the electrode andconsequently the salt bath which is in contact with the electrode. Theresistance element is maintained in circuit with a source of currentsupply until the bath has been heated to a temperature suiiicient topermit a predetermined current to flow through the bath and between theelectrodes disposed therein, after which, the circuit through theresistance element is opened automatically. Cur- 'rent from the sourceofv supply will continue to flow through the bath between the electrodesin a known manner for maintaining the bath at any desired operatingtemperature.

A preferred structural feature is a resistance element which extendssubstantially the entire length of that portion of the electrode whichis disposed in the salt bath so that a substantial portion of the bathis heated and thus the bath is caused to be quickly brought to operatingtemperature.

Now in order to acquaint those skilled in the Y art with the manner ofconstructing and operating my invention, I shall describe in conjunctionwith the accompanying drawings specific embodiments of the invention.

In the drawings:

Figure l is a schematic diagram of my invention incorporated in asingle-phase circuit; and

Figure 2 is a schematic diagram of a'modiflcation of the inventionincorporated in a threephase circuit.

' Referring now to Figure l, I have shown a `single phase transformer ihaving a variable primary winding 2 and a secondary winding 3. Theprimary winding is connected to a current supply line through theinstrumentality of a suitable outlet box 4 subject to control by aswitch 5 for energizing ordeenergizing the transformer. The secondarywinding is electrically connected by the pair of leads E and 1 extendingfrom opposite terminals of the secondary winding to suitablev electrodes8 and 9, respectively, which are immersed within a salt bath I0.

Nearly all salt baths employed in metallurgical` processes have thecharacteristic of being substantially non-conductive electrically atnormal or room temperatures and which when heated to a fairly hightemperature are rendered conductive. For example,`sodium cyanide andpotassium cyanide which are used in case hardening have thecharacteristic noted.

It will be seen, therefore, that when the bath i0 is at room temperatureno current will ow between the electrodes 8 and 9.

In order to preheat the salt bath I0 to a temperature at which currentwill flow between electrodes l and 9. I provide suitable resistanceelements l2 and I3 disposed within longitudinally extending bores ll-llin the electrodes 8 and 9. respectively. The resistance elements i2 andI3 are embedded in a suitable` refractory I5, such as aluminum oxide ormagnesium oxide within the bores I4-I4 of the electrodes for spacing theresistance elements from the inside walls of the tubular electrodes.'I'he resistance elements I2 and I3 are electrically connected to thelower ends of the electrodes 8 and 9 and extend substantially the-entirelength of the portion of the electrodes which are immersed in the bathto provide a substantial heating surface for rapidly bringing the saltbath between the electrodes to a temperature at which a current willpass between the electrodes.

The resistance elements I2 and I3 are connected to the secondary winding3 by leads AI1 and I8, respectively, through a switch I9 and leads 20and 2I, respectively. When the bath is at room temperature and theswitch 5 is closed to energize the transformer I, current will flowthrough leads 6 and I9, the switch I9, which is also normally closed,the lead 2I, resistance element I3, electrode 9 and lead 1 to completethe circuit. A circuit through resistance element I2 each of thesecondary windings '35, 36, and 31 in will also be established throughthe leads 1 and I1, switch I9, lead 20, resistance element I 2,electrode 8, and the lead 6. The resistance elements will heat theelectrodes which in turn raise the temperature of the bath between theelectrodes. When the bath between the electrodes has been raised asufficient extent, the main operating circuit will be establishedbetween the electrodes 8 and 9 and leads 6 and 1. When the lattercondition has been established, a current transformer winding 23 willinduce a current in the relay winding 24 which will attract an armature25 and open the switch I9 and consequently the circuits of the heatingelements I2 and I3. The bath between the electrodes then becomes themain resistance element and the bath will be quickly brought tooperating temperature. The heating elements I2 and I3 are disconnectedfrom the source of current supply after they have performed theirfunction of heating the bath to a sumcient temperature to complete themain operating circuit and are thereby prevented from becomingoverheated.

The salt bath I9 is retained in a suitable receptacle 28 which issurrounded by any suitable heat insulating material 21 for preventingloss of heat.

A pyrometer 28 may be 'disposed in the bath and connected to suitablemeter 29 which is also connected to the line. for indicating thetemperature of the bath.

It will be obvious that if desired only one heating element need beemployed, for example, resistance element I2 and its circuit, but it ispreferable to provide both electrodes with heating means, as shown, torapidly pre-heat the bath and establish the main operating circuit.

In Figure 2, I have shown my invention as utilized with a source ofthree-phase current. A three-phase transformer 3l having primarywindings 32, 33, and 34 and secondary windings 35, 38, and 31,respectively, is connected to the line in any suitable manner. In thisembodiment of the invention three electrodes 38, 39, and 40 are immersedin a salt bath III'. The electrodes 38 and 48 are connected by leads 4Iand 42 to corresponding terminals of the secondary windings 38 and 31,respectively. The electrode 39 is identical with electrodes 8 and 9 ofthe previous circuit. Resistance element 48 of the electrode 39 isconnected to the secondary winding 3B by lead 41, through switch I9',shown in its open' position, lead 48, electrode 39, and lead 43. Theresistance element 46 with the switch I9' in its closed position willheat the bath between the electrodes until the main operating circuit is-established between the electrodes 38, 39, and 40 after which a currenttransformer winding disposed about the lead 43 will energize the relaywinding 5I and open switch I9 and consequently open the circuit of theresistance element 46 with the secondary winding 36.

In Figure l, the salt bath is shown with the salt bath between theelectrodes heated to the point where current will pass betweenelectrodes 8 and 9 and with the switch I9 about to be opened. In Figure2, the salt bath is at its operating temperature and switch andresistance element 46 out of circuit. In the latter embodiment theelectrodes 38 and 40 are simply bar electrodes.

While I have shown what I consider to be the preferred embodiments of myinvention, it will be readily understood that various circuits may bereadily devised for carrying out the objects of my invention. Forexample, in the latter embodiment, three electrodes each having aresistance element for preheating the bath may -be provided or ifdesired a greater number of electrodes may be employed. It will beunderstood.

therefore, that the present embodiments are merely illustrative and notlimiting since the advantages of my invention may be incorporated in anelectric furnace in various ways.

I claim:

1. In a salt bath furnace the combination of electrodes adapted to beimmersed in a saltbath, one or more of said electrodes being a heaterelectrode consisting of a hollow tubularI body, a resistorlongitudinally disposed in said tubular body, said resistor beingelectrically insulated from the walls of said tubular. body but inthermally conductive relation thereto, current supply means includingconnections for impressing electrical potential lsimultaneously upon theterminals of the resistor to energizel the same for melting the saltbath about the heater elec- -trode and upon the electrodes to passheating current through the-salt bath as soon as lt is molten betweenthe electrodes, a switch in said resistor circuit, and means responsiveto a predetermined iiow of heating current through said electrodes inseries with the molten salt bath, said switch being opened to cut offthe heater due to the response of said means.

2. An electrode for an electric furnace or the like comprising a tubularmetallic member closed at one end, a resistance element disposed in saidtubular member and electrically connected to the closed end thereof,land refractory material in the tubular member in which said resistanceelement is embedded to space the latter from the inside wall of saidtubular member, the resistance element being adapted to heat saidrefractory material which in turn heats said tubular member byconduction.

- LEE R. TITCOMB.'

I9' opened l,

